Polyurethane-urea elastomers are widely used in industry to fabricate molded products. These elastomers are typically formed by reacting an organic polyisocyanate with a compound having a molecular weight between 400 and 10,000 containing at least two Zerewitenoff active hydrogen atoms, such as a polyhydroxyl compound, and an aromatic diamine chain extending agent. Alternatively, the chain extender is reacted with an isocyanate-terminated polyurethane prepolymer. Such prepolymers are well known in the art. In the molding operation the rate of reaction of the chain extender, or curative, and the processibility of the reacting composition is critical. If the reaction proceeds too fast, the composition will set up or gel before the mold can be completely filled. On the other hand, if the reaction is too slow, cycle times become too long and the cost of the operation is excessive. Finding the right curative for polyurethane-urea elastomers in a particular molding operation has been the subject of intensive research in this field for many years.
Three techniques have been used to reduce the reactivity of aromatic diamines in order to produce polyurethane-urea elastomer molding formulations with improved processibility. One technique involves incorporating organic substituents on the aromatic ring to hinder sterically the amine functionality. Klebert, U.S. Pat. No. 3,428,610 (1969) and Weber et al., U.S. Pat. No. 4,218,543 (1980) describe taking this approach to the problem, the latter patent also discussing the importance of reaction rates in the so called "one-shot" reaction injection molding (RIM) systems where the polyisocyanate, polyhydroxyl compound and aromatic polyamine are all combined at once rather than using a prepolymer.
A second technique involves adding an alkyl substituent to the amine nitrogen which both sterically hinders the amine group and reduces the number of active hydrogens. An example of this approach is given by Huffaker et al, U.S. Pat. No. 3,846,351 (1974) with N,N'-dialkyl-p-phenylenediamine. The third technique for reducing activity of an aromatic diamine is through electronic deactivation of the ring. Meckel et al., U.S. Pat. No. 3,736,350 (1973) take this approach by introducing ester and halogen or alkoxy groups onto the ring.
Lorenz, U.S. Pat. No. 3,188,302 (1965) describes a diamine curative which takes advantage of both steric hindrance and electronic deactivation to reduce its reactivity. Representative of such material is 4,4'-methylene-bis(2-chloroaniline) (which has been widely used in the art and is known by its shorthand name "MoCA". MoCA has the additional advantage of remaining liquid for long periods in the supercooled state even though it has a relatively high melting point of 130.degree. C. This enhances its processibility. Unfortunately, as pointed out by Baron et al., J. Appl. Polym. Sci., 20, pp.285-6 (1976) the Occupational Safety and Health Administration has placed MoCA on a list of suspected carcinogens thereby stimulating considerable research for a suitable "drop-in" replacement. Several candidates are described by Baron et. al. and in a related patent of Cerankowski et al., U.S. Pat. No. 3,932,360 (1976) as alkylene glycol di-p-aminobenzoates. These curatives are made by reacting p-nitrobenzoyl chloride with an alkylene or cycloalkylene diol followed by reduction of the nitro groups to amine. Preferably the diol contains an odd number of carbons, and more preferably 3 or 5 carbons. All species are said to possess reasonable supercooling properties with the best candidate compared with the commercial MoCA being 1,3-propanediol di-p-aminobenzoate. The curative derived from 1,2-propanediol is said to have given poor elastomers and was not a good MoCA substitute. Baron et al. further concluded that the reduced reactivity of these compounds is attributable to electronic rather than steric effects.
The search to replace MoCA, which for some molding operations is considered too slow, is illustrated by the '543 patent cited above and by Chung et al., U.S. Pat. No. 4,222,955 (1980) who describe diamino alkylbenzoates, alkylbenzonitriles and alkylene bis(amino alkylbenzoates) as polyurethane curatives which are slower reacting than prior aromatic diamines but faster than MoCA which frequently requires a catalyst to shorten its reaction time.
Bis-p-aminobenzamides have been known for over two decades as possible chain extenders for polyurethane elastomers. German Offen. DE 1962602 (1971) discloses N,N'-hexamethylene-bis-p-aminobenzamide as a chain extender for an isocyanate-containing prepolymer. Chain extenders of this type would be expected to be extremely high melting and difficult to process. Much more recently, Australian Patent Application No. 22278/88 (1989) disclosed a broad class of reactive thickeners for various polymeric materials, these thickeners containing aminobenzoic acid derivatives are included isocyanates. Polyurethanes are listed among the polymers that can be thickened, and both mono- and diaminobenzoic acid derivatives are included in the thickeners, including bis-aminobenzamides, with or without alkyl or aryl substituents on the amide nitrogens. Suitable amines include ortho-, meta- and para-aminobenzoic acid derivatives, with specific examples named including 1,6-hexamethylenediaminodi-p-aminobenzamide, 1,2-ethylenediamine-di-p-aminobenzamide and the corresponding o-aminobenzamides. Poor physical properties would be expected for polyurethane-ureas cured with orthosubstituted aromatic amines, and there is the possibility that such materials could undergo side reactions on curing due to cyclization reactions of the ortho substituents (see Baron et al., supra).
Lesher, U.S. Pat. No. 3,761,509 (1973) discloses N,N'-alkylenebis(benzamides) having straight or branched chain lower alkyl substituents on the amide nitrogens, but the only aminobenzamides permissible are alkyl or dialkyl substituted amines which would not be suitable chain extenders. These compounds are stated to have endocrinological properties.